Motor vehicles, the drive and brake systems of which comprise one or more electric machines, are gaining increasingly in importance. Here the electric machine is often used as drive, provided that a battery of the motor vehicle has stored sufficient electrical energy. It is desirable, with a view to the range of the electric drive, for example, to charge the battery or batteries of the motor vehicle not only when the vehicle is stationary but also whilst the motor vehicle is in operation. If the electric machine is operated as a generator, it is possible when braking the motor vehicle to obtain electrical energy by converting the kinetic energy or rotational energy of the axle (and the wheels attached thereto), which is connected to the electric drive, and to store this in the battery.
The brake torque built up through the recuperation, that is to say the recovery of electrical energy by a generator, varies as a function of the speed of the motor vehicle and the battery level. In order to be able to apply sufficient braking deceleration in every eventuality, brake systems for hybrid vehicles, for example, which combine an electric drive with an internal combustion engine, comprise additional brake devices besides the electric-regenerative brake (the electric drive operated as generator). These additional brake devices, which are usually incorporated in two statutorily prescribed brake circuits, comprise electro-mechanical, hydraulic and/or electro-hydraulic friction brakes, for example.
The interaction, in particular, of hydraulic friction brakes and a generator, which is desirable owing to the low system costs and moderate system complexity, presents some technical challenges: for an efficient recuperation the maximum brake torque of the generator possible in any given driving situation should be utilized. Furthermore a transition (“blending”) between a braking of the motor vehicle by the braking deceleration of the generator and a braking of the motor vehicle by friction brakes should be possible without the occurrence of jerky variations in the braking deceleration or an unfavorable brake force distribution. It is also necessary to avoid giving the driver an unusual pedal feel due to the decoupling of the electric-regenerative braking by the generator from the brake pedal, which makes it difficult for him to assess what braking effect he has achieved.
DE 196 04 134 A1, which is incorporated by reference, proposes a method and a device for controlling the brake system of vehicles having an electric drive, in which, in a first range of the driver braking requirement, the brake torque is applied almost exclusively by the regenerative braking. Here the pressure which the driver generates by actuating the brake pedal is reduced, in that the fluid flowing into the wheel brake cylinders is returned to an accumulator chamber by a corresponding control of the outlet valves. A development of this brake system is disclosed, for example, by WO 2004/101308 A1, which is incorporated by reference.
The brake systems hitherto described have the disadvantage, however, that in filling the accumulator chamber or the pressure accumulator fluid acts upon at least one wheel brake in each brake circuit. There is therefore always some deceleration of the vehicle by the friction brakes, which limits the efficiency of the recuperation, that is to say the proportion of the energy recovered by the generator during braking is reduced.